Generally, because a permanent magnet (PM) synchronous motor is not self-starting, implementations of PM synchronous motors can include a frequency-change starting method using a variable-voltage variable-frequency (VVVF) solid-state inverter. When the speed control is not required, a rotor cage winding method can be more cost-effective solution to starting the PM synchronous motor because the VVVF solid-state inverter is not necessary. A rotor cage winding method equips the PM rotor with a cage winding to provide asynchronous starting and operation without the use of solid state converters.
A PM synchronous motor with asynchronous starting is sometimes referred to as a line start permanent magnet (LSPM) brushless motor. The LSPM brushless motor employs one or more permanent magnets (PMs) such that the interaction of the stator rotating magnetic field and the rotor currents induced in the single-cage winding produces a starting torque that rotates the rotor. After rotation is initiated, the rotor is pulled into synchronism and rotates with the speed imposed by the line input frequency. The efficiency of line start PM motors is higher than that of equivalent induction motors and the power factor can be equal to unity. Although a LSPM brushless motor does not require solid state converters, conventional line start LSPM motors rated at about 10 kW and above, may draw unacceptable high inrush current exceeding several times the rated current. For instance, a high starting current in the range of approximately 4 to 8 times higher than the rated current, for example, is typically necessary to generate the starting torque. In addition, the PMs included in the rotor assembly increases costs and the overall weight of the motor.